In recent years, size and weight reduction of mobile information terminals, such as cellular phones, notebook computers and PDAs, has rapidly progressed. Batteries serving as their driving power sources are being required to achieve a much higher capacity. Among various types of secondary batteries, lithium ion batteries having particularly high energy densities have increased the capacity over the years, but under the existing conditions cannot fully respond to the above requirement. In addition, recently, the application of lithium ion batteries has been expanded beyond mobile information terminals, such as cellular phones, to serve as middle to large size batteries for electric tools, electric cars or hybrid cars by taking advantage of their features. Thus, there has been a tremendous increase in the demand for further increasing the capacity and power of lithium ion batteries.
There has recently been disclosed a technique of increasing the capacity and power of a battery by increasing the end-of-charge voltage from 4.1-4.2 V (4.2-4.3 V as a voltage versus the potential of a lithium reference electrode (vs. Li/Li+)) that would conventionally be used to 4.3 V or more (4.4 V (vs. Li/Li+) or more) to increase the utilization factor of the positive electrode (see Patent Document 1).
For the purpose of increasing the battery capacity, consideration has been made of high-density packing of electrode material, thickness reduction of a current collector, a separator or a battery housing that are members uninvolved in power generation factors, and other measures. On the other hand, for the purpose of increasing the battery power, consideration has been made of increasing the electrode area, and other measures. In terms of battery construction, challenges of electrolyte permeability into each electrode and electrolyte retentivity of the electrode are being given more attention today than in the early days of development of lithium ion batteries. It has become necessary, in establishing a novel battery construction, to solve the problems as thus far described in order to ensure the battery performance and reliability.
A technique is disclosed in which, in order to solve the above problems, a porous layer having an excellent nonaqueous electrolyte permeability is disposed between at least one of the positive and negative electrodes and a separator and allowed to function as a diffusion path for supplying an electrolytic solution present in a remaining space of the battery to the interior of the electrode, thereby improving the battery characteristics (see Patent Documents 2 and 3). When the positive electrode is charged to above 4.40 V versus the potential of a lithium reference electrode, the electrolytic solution may be likely to be oxidatively decomposed to largely reduce the amount of electrolytic solution in the battery. The above technique acts more effectively under such a condition and, therefore, is a useful technique for increasing the capacity and power of a battery.
The inventors have considered, as a porous layer to be disposed between at least one of positive and negative electrodes and a separator, a porous layer made of inorganic fine particles and a resin binder, and have considered, as the resin binder, polyamide, polyimide, polyamideimide or like resin.
Techniques using polyamide, polyimide, polyamideimide or like resin for a separator have already been considered for the purpose of increasing the heat resistance (see Patent Documents 4 to 7). In these conventional techniques, however, the resins have been considered simply focusing on improving the safety.    Patent Document 1: Published Japanese Patent Application No. 2006-147191    Patent Document 2: Published Japanese Patent Application No. 2007-123237    Patent Document 3: Published Japanese Patent Application No. 2007-123238    Patent Document 4: Published Japanese Patent Application No. H10-6453    Patent Document 5: Published Japanese Patent Application No. H10-324758    Patent Document 6: Published Japanese Patent Application No. 2000-100408    Patent Document 7: Published Japanese Patent Application No. 2001-266949